1. Field of the Invention
The present invention relates to a gallium nitride based light emitting element fabricated by using a gallium nitride substrate and, more particularly, to an improvement of an electrode of the light emitting element.
2. Description of the Background Art
Most of the conventional gallium nitride based blue light emitting elements, which have already been practically used, are fabricated by using sapphire substrates. A sapphire substrate, however, is insulative, and therefore, it is necessary to form positive and negative pad electrodes for wire bonding on one same surface of the substrate. This imposes a limit in an attempt to increase the number of chips that can be obtained per one substrate wafer, by reducing the size of the light emitting element chip.
Blue light emitting elements using n type SiC substrates have also been practically used. Here, as a SiC substrate is conductive, it is unnecessary to form both the positive and negative pad electrodes for wire bonding on one same surface of the substrate, and therefore, it is possible to reduce the size of the light emitting chip. There is a lattice constant difference, however, between the SiC substrate and the gallium nitride based semiconductor layer stacked thereon. Therefore, it is necessary to form a buffer layer on the substrate and, in order to obtain a flat crystal surface of the underlying layer, it is necessary to form the underlying layer thick.
In order to solve these problems, the inventors have fabricated a light emitting element using a gallium nitride substrate that is conductive.
Here, as the substrate is conductive, it is unnecessary to form both the positive and negative pad electrodes for wire bonding on one same surface of the substrate, it becomes possible to reduce the size of the light emitting chip, and hence it becomes possible to increase the number of chips obtained per one substrate wafer. Further, there is almost no lattice constant difference between the gallium nitride substrate and the gallium nitride based semiconductor stacked thereon. Therefore, the buffer layer becomes unnecessary and formation of a thick underlying layer becomes unnecessary as well.
Though a light emitting element using a gallium nitride substrate has been realized, a phenomenon is observed in the light emitting element that optical output lowers when conduction is continued for a long period of time.
Therefore, an object of the present invention is to provide, with satisfactory production efficiency, a gallium nitride based blue light emitting element having high brightness and high reliability.
The gallium nitride based light emitting element in accordance with the present invention includes an n type electrode formed on one main surface of an n type gallium nitride substrate, the ratio of the area of the n type electrode with respect to the area of the one main surface is set in the range of at least 5% and at most 60%, and the n type electrode includes an n type ohmic electrode layer for introducing current.
The n type electrode may further include a supplemental electrode layer that is in electrical contact with the n type ohmic electrode layer. The ohmic electrode layer and the supplemental electrode layer may have different two-dimensional shapes.
The ohmic electrode may be formed of a metal selected from Ti, Sc, La, Y, Hf and Zr. The supplemental electrodes may be formed of a metal selected from Pt, Al, Ag, Au and W.
It is preferred that the thickness of the n type gallium nitride substrate is at least 40 xcexcm. It is preferred that the thickness of the n type electrode formed on the substrate is in the range of at least 0.3 xcexcm and at most 2 xcexcm.